Following a series of strange events at an archeological excavation in Iraq, including the discovery of a Christian amulet and an ominous statue of the demon Pazuzu, aging Jesuit priest Father Merrin accepts that he will soon be called on to perform an exorcism. Meanwhile, movie star Chris MacNeil is on location at Georgetown University, a Jesuit school near Washington, D.C., where she lives in a rented two-story house with her ten-year-old daughter Regan, secretary Sharon, butler Karl and housekeeper Willie. Already fearful that her recent estrangement from Regan’s father might cause her daughter emotional damage, Chris is concerned when Regan claims that “Captain Howdy,” an imaginary person, answers the questions she presents to a Ouija board game. Elsewhere on campus, Father Damien Karras, an Ivy league-educated psychiatrist who counsels clergy, is having a crisis of faith. Each day signs of human suffering confirm his doubts, including an encounter with a derelict beggar and visits to his ailing mother, a Greek immigrant who suffers alone, unable to leave her apartment because of an injury. One night, Regan, who earlier heard her mother spout obscenities about her absent father, climbs into bed with Chris, claiming that her own bed was shaking violently. Unable to sleep, Chris hears growling sounds from the attic and is frightened when her candle is blown out while she investigates. When Chris takes Regan to the hospital for a physical soon after, the young girl is uncharacteristically aggressive and blasphemous, but Dr. Klein assures Chris that Regan is merely suffering from depression caused by her parents’ separation and prescribes Ritalin. Meanwhile on campus, a priest discovers that the chapel’s statue of the Virgin Mary has been defaced with red paint and makeshift breasts and penis. Karras is angst-ridden after his uncle insinuates that Karras’ choice to become a penniless priest rather than a well-paid psychiatrist has forced the family to put his mother in a mental institution. Visiting the facility, Karras is devastated when his mother refuses to speak with him. Late one night during a celebrity cocktail party held at the MacNeil house, Regan descends the stairs in her nightgown and, gesturing toward a guest, announces that he will die then urinates on the floor. Chris puts her scared daughter to bed and assures her that the strange behavior is just bad nerves; however, later that night, Chris, finds a screaming Regan being violently tossed on the bed, moving as if controlled by another force. Days later, Father Dyer comforts his close friend, Karras, who is tortured with guilt after his mother dies. Meanwhile, Regan’s unexplained illness worsens, prompting Chris to take her to the hospital where she screams profanities, spits and fights. Attributing Regan’s personality change to temporal lobe lesions, Klein takes brain scans and consults with specialists. Called to the house on emergency one night, Klein and brain expert Dr. Tanney find Regan being slammed against her bed by an unexplained force while her eyes roll back in her head and she rages in a sub-human voice, “fuck me.” After Regan’s slap sends Klein hurtling across the room, the doctors sedate her and suggest to Chris that Regan’s pathological state has given her increased motor power. When a battery of excruciating tests fails to reveal that anything is physically wrong with Regan, Klein admits that the cause must be psychological. One evening, Chris returns home after work to find the lights blinking on and off and Regan alone in a freezing cold bedroom with the window open. Sharon arrives moments later and explains that she left Regan with family friend and the film’s director Burke Dennings, who has mysteriously disappeared. They soon learn that Burke has been found dead, having apparently broken his neck while falling down the stairs outside Regan’s window. That evening, Regan contorts into a bizarre backbend and races backward down the stairs, spewing blood and horrifying her mother, who now suspects Regan is responsible for Burke’s death. Days later, when a psychiatrist hypnotizes Regan to address the other, apparently beastly person inside her, Regan grows pale and growls, then exhales a noxious breath and grabs the psychiatrist’s genitals. Meanwhile, homicide detective William F. Kinderman, who suspects that the chapel desecration and Burke’s death are linked, questions Karras about witchcraft, a subject about which the psychiatrist has written about. Kinderman explains that Burke’s head was turned completely around, facing backwards, an unlikely position to be caused by the fall. Kinderman suspects that Burke was murdered and that the perpetrator might be a mentally ill priest rebelling against the church, but Karras reminds him that confidentiality precludes him from revealing any information. After Chris adamantly refuses to institutionalize Regan, the doctors recommend a Catholic exorcism to rid Regan of the invading spirit, causing Chris to leave, insulted by the suggestion of witchcraft, although she knows she cannot keep Regan sedated at home forever. Interviewing Chris about Burke’s death days later, Kinderman states that Regan was obviously not powerful enough to push the grown man out of her window, but wants to speak with Regan about whether someone else was in her room that night. Chris is barely able to keep her composure, so certain is she that Regan is probably responsible, but manages to usher Kinderman out. Minutes later, a terrified Chris finds Regan stabbing her genitals with a crucifix and screaming, “let Jesus fuck you.” Regan then slaps Chris across the room, blocks her exit by moving the furniture with sheer will and, in Burke’s voice, states that Regan killed him. Desperate, Chris introduces herself to Karras, in hopes that he might perform an exorcism, but the priest tells her that the church has not had call to use the ritual in many years and must have sufficient proof of the devil’s possession before sanctioning an exorcism. Karras reluctantly visits Regan, who is so wounded with gashes from self-abuse that her feet and hands are now tied to the bed, while the furniture has been padded to prevent further injury. Seeing the priest, Regan introduces herself as the devil in the beast’s voice and uses the voice of the derelict beggar to intrigue him further. When Regan insinuates that Karras’ mother is among the inhabitants of Regan’s body, Karras asks for her maiden name, prompting Regan to vomit on him. Although doubtful of the possession, Karras cannot explain Regan’s knowledge of his mother’s death nor her ability to speak Latin, French and an indiscernible language. After Regan writhes as if burned when Karras sprinkles her with holy water, he informs Chris that the water was actually from the tap and that he is unable to support a case for exorcism, prompting Chris to admit that Regan pushed Burke to his death. Karras then listens to tape recordings of Regan’s voice played backwards, which reveal different voices calling for Regan’s death. Finally, after Sharon shows him raised welts on Regan’s body that spell out “Help Me,” Karras presents the case to the president of the university, who summons Merrin to perform the exorcism. The night he arrives at the house, Merrin warns Karras, who is acting as his assistant, to avoid conversing with the devil, who will lie to them to avoid the confrontation. When Karras tries to explain the case’s background, Merrin silences the priest, stating that they deal only with the present. As they enter the room calling out Regan’s full Christian name, the priests can see their breath in the frigid air. Merrin sprinkles holy water and recites the Lord’s prayer while Regan, now grotesquely deformed by the beast within her, screams profanities at the men and hisses at them with a long serpent tongue. Although Merrin proceeds undeterred, Karras is shocked speechless but soon joins Merrin in chanting vehemently for the devil to be cast out of Regan’s body. The room shakes and the walls crack, while Regan accuses Karras of killing his own mother, turns her head completely around and busts her restraints. As she levitates, the priests chant in unison, yelling, “the power of Christ compels you,” which quells the creature. While taking a break, Karras asks Merrin why the devil would inhabit such an innocent girl, and Merrin replies that he wants to make us believe that humans are merely ugly, worthless animals whom God could not love. Physically exhausted by the events, Merrin secretly takes nitroglycerine pills to stave off a heart attack and returns to the room to find Karras falling victim to the devil’s lies and sends Karras away. Minutes later, when Chris asks him if Regan is going to die, Karras regains his strength and returns to the room, but finds Merrin unconscious and Regan freed from her restraints. Unable to revive the priest, Karras chokes Regan, raging for the devil to take him instead of the girl. When Regan rips the medallion from Karras’ neck, the devil enters Karras’ body, leaving Regan to collapse on the floor, whimpering. Karras drives the demon out of his own body just before hurtling himself out the window. The detective, who has just arrived, rushes to the room to find Merrin dead and Karras at the bottom of the stairs. Hearing her daughter’s voice, Chris rushes to aid Regan, while at the foot of the stairs, Dyer asks the dying Karras to confess his sins so that he might enter heaven. Days later, Dyer arrives at the house just as Chris and Regan are about to move out. Regan, who remembers nothing of the possession, sees the priest’s collar and kisses him. When Chris then attempts to give him Karras’ medallion, Dyer hands it back as a memory of their faith. As the MacNeils drive away, Kinderman, who will not press charges against Regan, once again tries to befriend a priest, this time Dyer, to get to the bottom of the strange events.